Artificial reverberation apparatus



May 2, 1961 Filed 001- 15, 1958 H. E. MEINEMA ETAL ARTIFICIALREVERBERATION APPARATUS 2 Sheets-Sheet 1 vOOO SOUIZCE OF ELECTIZ l CALMUS! CAL TONE SIGNALS AND 0 VOLU M CONTROL 6 EEVERBERAT\ON APPARATUSAMPLIFIER I May 2, 1961 H. E. MEINEMA ETAL 2,932,819

' ARTIFICIAL REVERBERATION APPARATUS Filed Oct. 13, 1958 2 Sheets-Sheet2 ARTIFICIAL REVERBERATION APPARATUS Herbert E. Meinema, Lake Forest,and Herbert H. Canfield, Elmhurst, Ill., assignors to Hammond OrganCompany, Chicago, 111., a corporation of Delaware Filed Oct. 13, 1958,Ser. No. 766,840

16 Claims. (Cl. 179-1) The invention relates generally to apparatus formechanically and electrically introducing reverberation effects inmusic, approximating desirable acoustic reverberation, particularly foruse in electrical musical instruments, phonographs, tape recorders, and,in general, in a sound signal transmitting system in which the signalsource lacks reverberation.

It is an object of the invention to provide an improved reverberationapparatus which closely simulates acoustic reverberation effects, inwhich disadvantageous cancellation and reinforcement of direct andreflected signals is minimized.

A further object is to provide an improved compact reverberationapparatus in which the electric sound signal is mechanically transmittedthrough a plurality of coiled wire springs of different lengths, andemploying similar driver and pickup means at the ends of the springs.

A further object is to provide improved electromagnetic drivers andpickups.

A further object is to provide a reverberation apparatus having improvedlocking means for the mechanism to prevent damaging of the apparatus bymechanical shock.

A further object is to provide a helical coiled Wire as a transmitter ofsound vibrations, in which the sound is transmitted as a torsionalvibration and in which compensation is made for the concomitantcompressional modulation by the sound signal transmitted. A furtherobject is to provide an improved means for partially damping the soundvibrations at the ends of the springs.

Other objects will appear from the following description, referencebeing had to the accompanying drawings in which:

Fig. 1 is an elevational view of the reverberation apparatus, shownapproximately half-size;

Fig. 2 is a side elevational view thereof;

Fig. 3 is an enlarged plan view of one of the electromagnetic driver (orpickup) assemblies;

Fig. 4 is a vertical sectional view taken on the line 4-4 of Figure 3;

Fig. 5 is a greatly enlarged view (approximately ten times) of thearmature magnet support assembly of an individual pickup or driver;

Fig. 6 is a fragmentary sectional View of the central portion of one ofthe springs showing the manner in which the two portions of the springare joined;

Fig. 7 is an enlarged fragmentary sectional view taken on line 7-7 ofFig. 1;

Fig. 8 is an enlarged fragmentary sectional view taken online 8-8 ofFig. 1;

Fig. 9 is an enlarged fragmentary sectional view taken on line 99 ofFig. 1, and;

Fig. 10 is a block diagram illustrating a typical system in which thereverberation apparatus may be used.

The art of delaying sound transmission by imparting torsional orcompressional vibrations at one end of a coiled spring and picking upthe vibrations by a mechanico-electric transducer at the other end ofthe spring is well known. (See patent to R. L. Wegel No.

States Patent Of 2,982,819 Patented May 2, 1961 1,852,795.) It is alsoknown that such sound delay spring may be used as a reverberationapparatus by causing the sound to be repeatedly reflected at the ends ofthe spring, and by providing means to damp the vibrations of the springto cause gradual decay in the amplitade of vibration, and thus simulatethe effect of reverberation. (See patent to Laurens Hammond No.2,230,836.)

Because of the finite effective length of the spring it is highlyprobable, that, in transmitting sound vibrations of a certain frequencyor frequencies, the reflective sound wave will return to theelectromechanical input driver in phase opposite to that of the signalwhich is electrically supplied to the driver and will thus partiallycancel the input signal or, depending upon the relative phases of thereflected and driving signals, there may be an impairment of theultimate sound signal picked up by the electromechanical transducerranging from nearly total cancellation to total reinforcements. Thus,the frequency response curve of the apparatus as a whole will not besmooth but instead have deep valleys where the reflected and directsignals are out of phase and peaks at such frequencies at which thedirect and reflected signals at the pickup are more or less in phase.This non-uniformity in frequency response is particularly noticeable andvery undesirable at the lower frequencies.

In acoustic reverberation, as heard when music is played, especially ina large room, the sound from the source is reflected by the Walls,ceiling, and floor of the room and by objects in the room so that themusic reaches the listeners ear through a large, practically infinite,number of paths. These multiple reflections continue until the soundenergy is gradually absorbed by objects of poor sound reflectioncharacteristics and by the surfaces of the objects which reflect thesound, causing a gradual reduction in sound volume or intensity to alevel below the lower threshold of hearing.

Thus, an ideal electromechanical reverberation apparatus should likewiseinclude a substantially infinite number of points at which sound wavesmay be reflected and thus provide a large number of paths by which thedirect end reflected sound waves are transmitted to the car. In view ofthe impracticality of providing a very large number of points ofreflection of the sound energy, and a corresponding large number ofpaths for transmission of the sound, and means absorbing the soundenergy, a reasonable number of such paths and points of reflection andabsorption of the sound has been selected.

In said Hammond patent the sound is transmitted to the pickup throughonly one path (spring) although there are quite a few points at whichthe sound vibration may be reflected, and in this patent there are shownthree means for damping or absorbing the sound energy. It has been foundthat transmitting the sound energy from the driver to the pickup throughtwo springs greatly improves the overall frequcncy response of theapparatus, but by providing three coil springs forming paths of soundtransmission between the driver and pickup, the frequency response curveof the apparatus as a whole is startlingly improved as compared with theapparatus using two springs. Use of more than three springs does notnotably improve the results obtainable.

As shown in the drawings, the reverberation apparatus of this inventioncomprises a mounting channel 20 (Figs. 1 and 7) having keyhole slots 22therein for facilitating its attachment to a musical instrument case orcabinet. A driver, pickup and spring supporting frame 24 is suspendedfrom the channel 20 by a pair of tension coil springs 26. These springsform antishock mounts so as to minimize the transmission of externallyinduced mechanical vibration to the frame 24. The support frame has acentral downwardly extending channel member 28 secured thereto by studs29 (Fig. 7). The lower end portion of the channel (Figs. 1, 8 and 9) hasits flanges cut away and extends through a rectangular hole 31 in asuitable bracket 36, the lower extremity of the channel 28 floatingbetween two tension coil springs 32 suitably anchored to the bracket 30.The latter may be secured to any stationary part of the instrumentcabinet, or the like, with which the apparatus is used. The springs 32maintain the channel end part centered in the hole 31 and also serve todamp the transmission of vibration from the cabinet through the channel28 to the vibration sensitive portions of the apparatus.

A driver assembly, designated generally by the reference character 36(Figs. 1 and 4), and a pickup assembly 37 are suitably mounted on theframe 24 and serve as end supports for three helical coil springassemblies 40, 41, and 42 which naturally assume the catenary shape. Asshown in Fig. 6, each of the springs is, in effect, two springs such as43a and 4% which have their lower ends, bent to extend axially of thecoil, and secured to each other by a coupling sleeve 48, as by solderingor brazing.

The two portions 49a and 40b of the spring as a whole are wound inopposite directions to compensate for the vibrations transmitted in thecompressional mode. As willbe more clearly described hereinafter, thesound vibrations are transmitted through the springs in the torsionalmode, but as a result of such torsional undulations of the springs thesound also tends to be transmitted in the compressional mode. Thus, asone section of the spring, due to the slight winding up thereofresulting from the torsional vibration, the other section of the springwill be unwinding. Because the sections of the springs are of equallengths for each spring there is a net cancellation of compressionalvibration of -the spring and it does not have any effect either at thedriver or at the pickup. Vibrations transmitted in the torsional modetravel along a coil spring at a velocity differing from that at whichcompressional vibrations travel and if the compressional vibrations werenot cancelled out distortion would result.

The frame 24 is provided with a pair of rubber-like grommets 50 (Figs. 1and 7) through the enlarged hole of which project a pair of studs 52respectively, which are secured to a downwardly extending flange 53 ofthe channel 26.

Means are provided to clamp the frame and the springs to prevent damagein shipment. This means comprises a shaft 60 (Figs. 2 and 7) suitablyjournaled in bearing brackets 61 secured to the frame 24, has plateearns 62 and 64 rigidly secured thereto near its ends, which, in theposition shown in Figs. 2 and 7, engage a metal strip 66 mounted forlateral movement on a pair of studs 68 secured to frame 24, and is heldin contact with the cams by compressed coil springs 70. The end portionsof the strip66 are offset to the right (Fig. 2) and have cementedthereto strips of rubber 72, which are of sufiicient length to overliethe upper ends of the springs 40, 41, and 42. Corresponding strips ofrubber 74 are cemented to the frame 24 to the left (Fig. 2) of the strip66. The rotation of the cams 62. and 64 is limited to 90 by detentspring means 76 carrying a suitably guided pins 77 engageable with 90spaced limit stop lugs on the cams.

The shaft 60 is provided with a kerf at one end so as to be rotatablyadjusted by a screwdriver. When it is rotated counterclockwise 90 fromthe position shown in Fig. 2, the springs 70 cause the ends of thesprings 40, 41, and 42 to be clamped between the felt pads. Engagementof the lobes of the cams with flanges 71 on the channel 2i) causes theframe to be moved downwardly and tilts it slightly so that the lower endof the channel 28 is pressed against the edge of the hole 31 In thebracket 30. This occurs as downward movement 4 of the frame is limitedby engagement of the grommets 51) with studs 52.

The driving assembly 36 (Figs. 1, 3, and 4) is substantially the same asthe pickup assembly 37 in its mechanical construction and only thedriver assembly will be described in detail. This assembly comprises anangle bracket 78 suitably secured to the frame 24. The generallyhorizontal part is provided with three perforations for the reception ofthe three collars 80 which are swaged to the bracket. Each of thesecollars is provided with an internal shoulder 32 against which a dampingdisk 84 is pressed by a second sleeve 86, the latter having a hollowconical metallic cap 83 pressed thereon.

This cap has an aperture to receive the end of an armature supportingwire 90, which is soldered to the cap 88.

As best shown in Fig. 5 the wire 9%, which is preferably made of acopper-beryllium alloy, extends through a length of tubing 92. The lowerend portion of the tubing 92 is flattened and formed into a hook 4. Thewire preferably extends through at least a portion of the hook portion94 and is soldered to the tube 92. The hollow cylindrical magnet 92 ispolarized in a direction parallel to the pole faces 98 of a generallyU-shaped laminated core structure 100 which has two relatively longparallel legs and is suitably secured to the bracket 78 by bending overtabs 162 of an apertured channel-shaped clamping member 103. A suitablyperforated insulating sheet 164 is clamped between the core and thehorizontal portion of the bracket 78, and has soldering lugs 79 rivetedthereto. A pair of coils 196, one over each leg of the core structureare wound so as to be effective in the same direction to produceadditive electrical results. Two coils, rather than one, are provided inorder to reduce pickup of stray magnetic fields.

As above mentioned, the permanent magnets 96 are magnetizeddiametrically, in a plane parallel to the inner pole faces 93 of thecore 1%. The magnetization is perpendicular to the direction in whichthe hook portion 94 is bent. The magnet may be of any suitable materialof high retentivity, preferably a ceramic material of the ferrite group.The rotary moment of inertia of the magnet 96 is matched to thecompliance of its support wire and also matched to one turn of thespring which is attached to the hook portion 90.

The damping washers 84, are made of a so-called dead rubber or the like,that is, a rubber-like composition which has internal friction operatingto slow down its return to normal shape after it has been distorted.These washers are pierced by the magnet supporting wires 90 and havediffering thicknesses so as to apply successively greater damping as thelength of the spring, with which it is associated, increased. The reasonfor this is that a longer time is taken for transmission of the signalthrough the longer coil spring wires than through the shorter ones and,since it is desirable that the signais transmitted and reflected throughthe three springs should decay at about the same rate, the extent ofdamping must be greater as the length of the spring increases. The endsof the three composite spring have their last half turn bent at rightangles in the usual manner, and these ends are hooked into the portions94 and thus, as previously indicated, are suspended in catenary shape.The brackets 78 are mounted at an angle to the horizontal to accommodatethe manner of spring suspension.

The central portions of the composite springs 40, 41, and 4-2, extendthrough U-shaped wire Spring clips lit the ends of which are pressedinto suitable perforations 112 formed in the vertical channel 28 toprevent the springs from swinging too far if the support for theapparatus is tilted.

As previously noted, the pairs of spring portions which are joined asshown in Fig. 6 are wound in opposite directions and that the ends ofthese springs which project into the connecting sleeve 43 are coaxialwith the of the springs so that this joint is capable of torsionaltransmission of the sound vibrations. Since the compressional modulationof the spring is balanced by the fact that the two sections of thespring are Wound in opposite directions the torsional vibration inputsupplied by the magnets 96 of the drivers are transmitted solely astorsional impulses to the magnets 96 of the pickup as sembly.

The fact that the musical tone signal is transmitted from driver topickup through three unique paths make it highly probable that theoutput frequency response curve will be very smooth and this probabilityis enhanced by the fact that the springs are of different lengths whichare not integral multiples of some whole number. Generally speaking, itis desirable that the attenuation provided by the damping disks 84 beapproximately 8 decibels per reflection for the spring 42, 6 decibelsfor the spring 41, and 4 decibels for the spring 40. The springspreferably have the same propagation velocity and the same impedance andare mechanically identical except for their length and number of turns.

It will be noted that the driver has constant current drive which yieldsa constant velocity drive in a mechanical spring system and in thepickup. The constant velocity at which the pickup magnets are oscillatedresults in the production of a constant voltage in the reactive output,With respect to frequency. This is a further important factor whichresults in a smooth frequency response curve in the output.

The coils 'of the driver assembly are, of course, matched in impedancewith the source of signal input while those in the pickup are impedancematched with the input impedance of the amplifier or other device towhich the signal is to be supplied.

Because of the use of three spring transmission paths the decay time maybe extended to as long as two seconds with pleasing results, but to theaverage listener a decay or reverberation time of about one and one-halfseconds is preferable. The three path transmission system insures thatthe apparatus, as a whole, will not be resonant at any frequency.

The apparatus of the invention has many useful applications. One typicaltype of circuitry in which this apparatus may be used is shown in Fig.10' and comprises a source of electrical musical tone signals 12!) whichhas its output coupled to a preamplifier and volume control 122, theoutput of the latter being supplied to a speaker 124. Also connected tothe output of the preamplifier 122 is the reverberation apparatus 126disclosed in detail in this application. The output of the reverberationapparatus is coupled to an amplifier 128 which feeds a speaker 130.Suitable switching means will usually be provided so that the signalsmay or may not be transmitted through the reverberation apparatus 126and such switching means may also be utilized to couple the amplifier128 to a suitable stage in the preamplifier 122 or to the output of thesource 120 so that the intensity of the sound output will beapproximately the same whether or not the reverberation apparatus is inuse.

The source of electrical musical tone signals 122 may comprise anelectric or electronic organ, a microphone, a phonograph, a taperecorder, or, in fact, any source of sound may have the reverberationeffect added to it by the apparatus of this invention.

While we have shown and described particular embodiments of ourinvention, it will be apparent to those skilled in the art that numerousmodifications and variations may be made in the form and constructionthereof, without departing from the more fundamental principles of theinvention. We therefore desire, by the following claims, to includewithin the scope of our invention all such similar and modified forms ofthe apparatus disclosed, by which substantially the results of theinvention may be obtained by substantially the same or equivalent means.

We claim:

1. An apparatus for providing reverberation of an electrical signalcomprising electro-mechanical driver means, said driver means having anelectrical circuit with input electrical terminals therefor andmechanical connections for a plurality of coil springs, said electricalcircuit operating to vibrate said mechanical connections when an audiofrequency signal is applied to said input terminals, mechanico-electricpickup means, said pickup means having mechanical connections for aplurality of coil springs and an electrical circuit having outputelectrical terminals, said pickup means operating to generate anelectrical signal and apply the last said signal to saidoutputelectrical terminals when its mechanical connections are vibrated,and a plurality of coil springs individually connected to the mechanicalconnections of said driver means and to the mechanical connections ofsaid pickup means, said springs having appreciably different effectivelengths with respect to their rate of vibration transmission.

2. An apparatus for providing reverberation of an electrical signalcomprising electro-mechanical driver means, said driver means having anelectrical circuit with input electrical terminals therefor andmechanical connections for a plurality of coil springs, said electricalcircuit operating to vibrate said mechanical connections torsionallywhen an audio frequency signal is applied to said input terminals,mechanico-electric pickup means, said pickup means having mechanicalconnections for a plurality of coil springs and an electrical circuithaving output electrical terminals, said pickup means operating togenerate an electrical signal and apply the last said signal to saidoutput electrical terminals when its mechanical connections are vibratedtorsionally, and a plurality of coil springs individually connected tothe mechanical connections of said driver means and to the mechanicalconnections of said pickup means, said springs having appreciablydifferent effective lengths with respect to their rate of vibrationtransmission.

3. An apparatus for providing reverberation of an electrical signalcomprising electro-mechanical driver means, said driver means having anelectrical circuit with input electrical terminals therefor andmechanical connections for a plurality of coil springs, said electricalcircuit operating to vibrate said mechanical connections when an audiofrequency signal is applied to said input terminals, mechanico-electricpickup means, said pickup means having mechanical connections for aplurality of coil springs and an electrical circuit having outputelectrical terminals, said pickup means operating to generate anelectrical signal and apply the last said signal to said outputelectrical terminals when its mechanical connections are vibrated, and aplurality of coil springs individually connected to the mechanicalconnections of said driver means and to the mechanical connections ofsaid pickup means, said springs having appreciably different effectivelengths with respect to their rate of vibration transmission, and eachof said springs being comprised of a pair of oppositely wound springelements joined in endto-end axial alignment.

4. An apparatus for providing reverberation of an electrical signalcomprising electro-mechanical driver means, said driver means having anelectrical circuit with input electrical terminals therefor andmechanical connections for a plurality of coil springs, said electricalcircuit operating to vibrate said mechanical connections torsionallywhen an audio frequency signal is applied to said input terminals,mechanico-electric pickup means, said pickup means having mechanicalconnections for a plurality of coil springs and an electrical circuithaving output electrical terminals, said pickup means operating togenerate an electrical signal and apply the last said signal to saidoutput electrical terminals when its mechanical connections are vibratedtorsionally, and a plurality of coil springs individually connected tothe mechanical connections of said driver means and to the mechanicalconnections of said pickup means, said springs if 7 having appreciablydifferent effective lengths with respect to their rate of vibrationtransmission, and each of said springs being comprised of a pair ofoppositely wound spring elements joined in end-to-end axial alignmerit.

5. An apparatus for providing reverberation of an electrical signalcomprising at least two coil springs having appreciably differenteflfective lengths with respect to their rate of vibration transmission,means supporting said springs at their ends to permit vibrations in 7said springs to travel the lengths of said springs and be reflected atboth ends of said springs, damping means connected to at least one endof each spring for controlling the rate of decay of spring vibration,electromechanical means having input electrical terminals and connectedto both said springs outwardly of an end of each spring to vibrate bothof said springs when an audio frequency signal is applied to saidterminals, and mechanico-electric pickup means having output electricalterminal and having connections outwardly of an end of each spring forproducing reverberative signals at said output terminals in response tovibrations in said springs. V

6. An apparatus for providing reverberation of an electrical signalcomprising at least two coil springs having appreciably differenteifective lengths with respect to their rate of vibration transmission,means supporting said springs at their ends to permit vibrations in saidsprings to travel the lengths of said springs and be reflected at bothends of said springs, damping means connected to at least one end ofeach spring for controlling the rate of decay of spring vibration,electromechanical means having input electrical terminals and connectedto both said springs outwardly of an end of each spring to vibrate bothof said springs torsionally when an audio frequency signal is applied tosaid terminals, and mechanico-electric pickup means having outputelectrical terminals and having connections outwardly of an end of eachspring for producing reverberative signals at said output terminals inresponse to torsional vibrations in said springs.

7. An apparatus for providing reverberation of an electrical signalcomprising at least two coil springs having appreciably differenteffective lengths with respect to their rate of vibration transmission,each of said springs being comprised of a pair of oppositely woundspring elements joined in end-to-end axial alignment, means supportingsaid springs at their ends to permit vibrations in said springs totravel the lengths of said springs and be reflected at both ends of saidsprings, damping means connected to at least one end of each spring forcontrolling the rate of decay of spring vibration, electro-mechanicalmeans having input electrical terminals and connected to both saidsprings outwardly of an end of each spring to vibrate both of saidsprings when an audio frequency signal is applied to said terminals, andmechanico-electric pickup means having output electrical terminals andhaving connections outwardly of an end of each spring for producingreverberative signals at said output terminals in response to vibrationsin said springs.

8. An apparatus for providing reverberation of an electrical signalcomprising at least two coil springs having appreciably differenteffective lengths with respect to their rate of vibration transmission,each of said springs being comprised of a pair of oppositely Woundspring elements joined in end-to-end axial alignment, means supportingsaid springs at their ends to permit vibrations in said springs totravel the lengths of said springs and be reflected at both ends of saidsprings, damping means connected to at least one end of each spring forcontrolling the rate of decay of spring vibration, electro-mechanicalmeans having input electrical terminals and connected to both saidsprings outwardly of an end of each spring to vibrate both of saidsprings torsionally when an audio frequency signal is applied to saidterminals, and mechanico-electric pickup means having output electricalterminals and having connections outwardly of an end of each spring forproducing reverberative signals at said output terminals in response totorsional vibrations in said springs.

9. Apparatus for providing reverberation of an electrical signalcomprising driver and driven units, a plurality of coil springsmechanically interconnecting said units, said coil springs havingappreciably difierent lengths with respect to the timetfor transmissionof vibrations from one end to the other, said driver and driven unitseach comprising means including an electric coil and a magneticstructure providing a magnetic circuit including a gap, a plurality ofmagnetically polarized armatures, one connected to each of said springs,positioned in said gap, a compliant wire member connected to each ofsaid armatures and extending away from its spring, means secured to andsupporting the end of each of said wire members, and damping meansconnected to each of said wire members adjacent its armature for dampingtorsional vibration of said wire.

10. Apparatus for providing reverberation of an electrical signalcomprising driver and driven units, a plurality of coil springsmechanically interconnecting said units, each of said springs beingcomprised of a pair of oppositely wound spring elements joined inend-to-end axial alignment, said coil springs having appreciablydifferent lengths With respect to the time for transmission ofvibrations from one end to the other, said driver and driven units eachcomprising means including an electric coil and a magnetic structureproviding a magnetic circuit including a gap, a plurality ofmagnetically polarized armatures, one connected to each of said springs,positioned in said gap, a compliant wire member connected to each ofsaid armatures and extending away from its spring, means secured to andsupporting the end of each of said wire members, and damping meansconnected to each of said wire members adjacent its armature for dampingtorsional vibration of said wire.

11. Apparatus for providing reverberation of an electrical signalcomprising driver and pickup units, a plurality of coil springsmechanically interconnecting said units, said coil springs havingappreciably different lengths with respect to the time for transmissionof vibrations from one end to the other, each of said springs comprisinga pair of oppositely wound spring elements joined in end- -toend axialalignment, said driver and pickup units each comprising means includingan electric coil and a magnetic structure having pole faces providing amagnetic gap, a plurality of armatures positioned in each of said gaps,each of said armatures being magnetically polarized transverselyrelative to its gap and connected one to each end of each of saidsprings, a compliant wire member connected at one end to each of saidarmatures and extending away from its spring, means secured to andsupporting the opposite end of each of said wire members, damping meansconnected to each of said Wire members adjacent its armature for dampingtorsional vibration of its wire member, and each of said springsextending away from both of its armatures in a direction to applytension to both of its wire members and center both of its armatures insaid gaps.

12. The combination set forth in claim 1 in which the springs areloosely suspended between the driver means and the pickup means so as todefine catenary curves.

v1?. The combination set forth in claim 5 in which the springs areloosely suspended between the electro-mechanical means and themechanico-electric means so as to define catenary curves. 7 a

14. The combination set forth in claim 9 in which th springs are looselysuspended between their wire members so as to define catenary curves.

15. The combination set forth in claim 11 in which the springs areloosely suspended between their wire members so as to define catenarycurves.

16. An apparatus for providing reverberation of an electrical signalcomprising electro-mechanical driver means, said driver means having anelectrical circuit with input electrical terminals therefor and amovable element, said electrical circuit operating to vibrate saidmovable element when an audio frequency signal is applied to said inputterminals, mechanico-electric pickup means, said pickup means having amovable element and an electrical circuit having output electricalterminals, said pickup means operating to generate an electrical signaland apply the last said signal to said output electrical terminals whenits movable element is vibrated, a vibration trans- 1O mitting coilspring mechanically connected to the movable element of said drivermeans and to the movable element of said pickup means, and said springbeing comprised of oppositely wound spring elements joined in endto-endaxial alignment.

References Cited in the file of this patent UNITED STATES PATENTS354,672 Barling Dec. 21, 1886 1,469,788 Hellenthal Oct. 9, 19232,104,811 Pfister Jan. 11, 1938 2,230,836 Hammond Feb. 4, 1941 2,437,445Stack Mar. 9, 1948 2,853,145 Martin Sept. 23, 1958

